Branched Path Tracing control for preview in 2.78
With branched path tracing the viewport preview quality is controlled by the AA Samples setting, to the left of the sample settings.

The amount of bounces is multiplied by the number of Anti Aliasing samples, thus controlling preview quality while keeping the same settings in sync for final render.

If those are not enough for you, you can alternatively add some custom presets on the menu above, keeping a lighter one for viewport preview, and a more complex one for final production.